Can't create pivot styles for multiple value columns.

**Do you want to request a *feature* or report a *bug*?**
- [x] Feature

**Version of ClosedXML**
0.95.0-beta1

**What is the current behavior?**
In the API pivotstyles there is a lack. We cannot assign different styles for different data fields.
```
nameField.StyleFormats.DataValuesFormat
.ForValueField(ordersField)
.Style.Fill.BackgroundColor = XLColor.Apricot;
nameField.StyleFormats.DataValuesFormat
.ForValueField(qtyField)
.Style.Fill.BackgroundColor = XLColor.ArmyGreen;
```
two styles should be created here, but it turns out that the same style will be changed

**What is the expected behavior or new feature?**
I suggest doing this:
```
nameField.StyleFormats.AddValuesFormat()
.ForValueField(ordersField)
.Style.Fill.BackgroundColor = XLColor.Apricot;
nameField.StyleFormats.AddValuesFormat()
.ForValueField(qtyField)
.Style.Fill.BackgroundColor = XLColor.ArmyGreen;
```

**Did this work in previous versions of our tool? Which versions?**
No

Start by tracing the StyleFormats.DataValuesFormat and ForValueField API shown in the issue. Compare the current behavior when formatting ordersField and qtyField, then evaluate the proposed AddValuesFormat entry point. Done means separate styles can be assigned to multiple value fields and the resulting workbook preserves both styles.

Written by the indexing model from the issue text.